Eudora OSE quickly died because the few core features it added had already been added to Thunderbird, it did a poor job of appealing to Eudora fans, and there was nobody left interested in maintaining it after the initial release. Basically it was Eudora in name only. Qualcomm closing the Eudora forums afterwards didn't help. Eudora OSE is based on Thunderbird 3.0.4 source code which is ancient. The Penelope extension (for Thunderbird) isn't even in Mozilla add-ons anymore. They're still on https://wiki.mozilla.org/Penelope_Extensions but its not clear whether they will work with a recent version of Thunderbird..